Due to the use of brake shoes and brake pads which are free of asbestos due to the health risks associated therewith, a significant problem has developed with regard to the useful life of the metallic components against which the shoes or pads bear. In heavy or high-speed vehicle applications where the thermal and normal stresses are high, this problem is accentuated. Whereas the previous asbestos brake materials for example would result in a brake drum life in a heavy vehicle of some 1.times.10.sup.6 km, the new asbestos-free brake materials might reduce the life, in certain cases, down to as low as 3.4.times.10.sup.5 km.
Further problems associated with brake drums and brake discs are brake squeal, brake judder and brake fade. Brake squeal is a noise which has a frequency of between about 600 Hz and 2000 Hz in drum brake applications and is particularly unpleasant for the vehicle driver as well as others in the audible area of the noise. This noise has been found not only to be related to the brake geometry but also to the materials of the brake drum/disc as well as the brake lining and has been found to be more intense with presently used friction lining materials.
Brake judder is a low frequency vibration of about 7 to 8 Hz and occurs due to the brake drum or brake disc being out of round, which is often an effect caused by built-in deformations from the process of wheel mounting and high temperature resultant thermal stress, which in the end may result in increased out-of-roundness in the brake component.
Brake temperature fade is a reduced braking effect caused, similar to brake judder, by increase in temperature. However, brake fade is due, in drum brakes for example, to the diameter of the drum increasing at a certain rate whilst the diameter of the shoe does not increase at a similar rate. The quality of the lining material also has an influence.
Several attempts have been made by others to overcome one or more of the above-mentioned problems. For example U.S. Pat. No. 4 948 437 and U.S. Pat. No. 4 961 791 disclose certain grey iron alloy compositions and specific subsequent heat treatments that have, as a combined process, been used to increase the tensile strength of the alloy, required after having raised the carbon content in the material to increase the conductivity and thereby reduce brake fade and judder. Clearly the use of a subsequent heat treatment adds to the cost and time of manufacture. Additionally the resultant alloy requires further improvement with respect to the aforementioned braking-associated problems.
A further array of possible alloy materials for disc brake applications is known from JP laid-open application no. 90-138438 which discloses a disc brake rotor made from an alloy developed to have high thermal resistance, high wear resistance and reduced brake squeal. Whilst the disc brake rotor according to that application may have certain advantages depending on the exact alloy composition chosen, the ranges given, such as for example the range of weight percentages for carbon (3.5 to 4.0%), for vanadium (0.02% to 0.35%), for copper (0.2 to 2%) and for molybdenum (0.4 to 1.2%), are very large indeed and cannot be said to give optimal results. Should any particular values inside these large ranges give an improved result, it is not stated what such restricted ranges or values might be. Indeed, the properties of the alloy obtained vary to a very large extent between the ends of the disclosed ranges.
The main object of the invention is thus to provide an alloy which is optimised for applications where the metal component is subject to wear by contact with a further component moving relative thereto.
In particular this object is directed to a brake drum or to a brake disc for automotive applications requiring optimised thermal resistance, squeal resistance and wear resistance having regard to the associated brake lining, with respect to which it is relatively movable (i.e. the relative movement being caused by the disc or drum rotation compared to the stationary friction linings of the pads or shoes). At the same time, the component must not be too hard since this would make machining more difficult, longer and thus more expensive.
A further object of the invention is to avoid the need for any subsequent heat treatment process.